A friend and I somehow came about to discussing polygamy, and why so many societies in the ancient world were polygamous. I thought that maybe the one of the reasons could be that if a large amount of the male population died prematurely from war and harsh physical labour, there could have been significantly more women than men in the society, making polygamy the optimal arrangement. But I can't find any sources on sex ratios on ancient/classical societies. Does anyone know anything about this?
Possibly war had something to do with it.
We don't have to rely solely on historical aaccounts - this thing predates history. Genetics studies of variability in our DNA show that we had approximately twice as many female ancestors as we had male ancestors. This shortage of male ancestry indicates that while most women were able to reproduce, far fewer men did. The social system that produces this result is polygyny, where relatively few men monopolize all the women for reproductive purposes. The genetic traces of this effect are far older than recorded history.
Unfortunately, without written records we are forced to speculate about causes. Wars that kill young men before they've had the chance to reproduce are a possible explanation, but it doesn't seem sufficient. Similarly, a risky lifestyle, perhaps hunting big game could take its toll on the male population, but again, it seems insufficient. The physical records that exist (skeletal remains) are patchy, but they don't support such high rates of attrition.
Social and economic factors must therefore have been at least partially responsible. An unequal system where some people received a lot of resources while others got very little, and where women were treated as resources. Historically, most political systems of the past are much less egalitarian than modern democracy.
It does seem likely though that wars to bleed off the surplus of young men were a kind of safety valve to keep society from being torn apart by an excess of young men who felt poorly treated.